This invention relates to an optical recording medium for making optical read and write of information by using an organic dye.
Photochromic materials are attracting attention for their availability for the erasable optical recording media owing to their specific property that they undergo a reversible change of color upon exposure to two types of light source differing in wavelength. Fulgides are known as a typical example of such photochromic materials. Fulgides are compounds having an alkylidene group bonded to each of the two methylene carbons of succinic anhydride or succinic imide thereof, and represented by the following general formula (1): ##STR2## (wherein at least one of R.sub.2 and R.sub.3 represents an aromatic ring, and Y represents 0 or ##STR3##
A large number of fulgide compounds have been known up to date. Among them, fulgide (2) is one of the fulgides having the most excellent photochromic properties (JCS Perkin Trans. I, 202, (1981)). This fulgide (2), when irradiated with ultraviolet light of 337 nm, is ring closed and converted into the red benzofuran form (3), but when the latter is irradiated with visible light of 473 nm, it returns to fulgide (2). ##STR4##
There are two problems in utilization of fulgide (2) for an optical recording medium. One problem is the presence of the regio isomer (4) which has no action of inducing photochromic reaction. Thus, the incidental formation of regio isomer (4) in the synthesis of fulgide (2) and the lowering of conversion rate of the principal reaction due to the side reaction of forming regio isomer (4) in the step of ultraviolet-light irradiation of fulgide (2) have been the pending questions. ##STR5##
In utilization of photochromic compounds for optical recording media, light quantum detector elements, etc., it is required to reduce the film thickness in correspondence to the miniaturization in size of electronic parts. Another problem is the impossibility to attain the desired thin films of fulgides by the Langumuir-Blodgett technique, which is an excellent method to produce ultra-thin organic films. For forming Langmuir-Blodgett films, it is necessary that a hydrophobic group (for example, hydrocarbon chain) and a hydrophilic group (for example, carbonyl group) be contained in a molecule. Since the introduction of a hydrophobic group into the molecule of fulgide (2) is attended with difficulties in the synthesis thereof, no successful attempt of such introduction has been reported.